Pre-emphasis, de-emphasis circuits are known for improving the signal to noise ratio in wide band signal transfer circuits. Johnson, U.S. Pat. No. 3,288,930, issued Nov. 29, 1966, describes circuits for pre-emphasis and de-emphasis of video signals. These circuits utilize diodes in a non-linear frequency-discriminating portion thereof to attenuate or accentuate the amplitudes of high-frequency, high-amplitude signals.
Diodes, such as used in the Johnson patent, have numerous disadvantages. It is difficult to match the characteristics of discrete diodes, which is a necessity for production of pre-emphasis and de-emphasis circuits utilizing such diodes. Diodes also have unknown capacitive components in their transfer characteristics which cannot be readily compensated for in the design of pre-emphasis, de-emphasis circuits.
The use of field effect transistors (FET) circumvents the disadvantages inherent in diodes, and they have additional advantages. The capacitive reactance of FETs is sufficiently small as to be negligible when the devices are utilized in pre-emphasis, and de-emphasis circuits. Also, the characteristics from one FET to another are sufficiently reproducible as to allow their use in production situations without the necessity for highly skilled personnel to match the characteristics of the components. A further advantage of the FET is the ability to control the operating point by simply varying the control gate voltage.
It is an object of this invention to produce pre-emphasis and de-emphasis circuits having improved control characteristics.
A second object of the invention is to eliminate the problematic capacitive reactance present with the utilization of diodes.
A further object of the invention is to develop circuits which can be constructed in a production situation without the necessity for skilled personnel to carefully match characteristics of the circuit components.